Vasoactive intestinal peptide (VIP), a 28 amino acid neuropeptide widely distributed in the mammalian nervous system, has potent neurotrophic actions that influence nerve cell function. In the central nervous system, this role of VIP is translated into developmental effects, display of growth factor activities and maintenance of neuronal survival and function. Neurons, which are capable of releasing VIP, innervate blood vessels throughout the body, as well as the trachea in the lung, and the released VIP serves as a potent vasodilator, inducing smooth muscle relaxation. Radioligand binding assays, pharmacological experiments, molecular cloning and development of superactive novel derivatives have indicated several classes of VIP receptor sites and several potential therapeutical uses.
Two possible therapeutical uses of VIP, modified VIP or lipophilic VIP derivatives were reported in our previous Patents IL 87055, EP 0354992 and U.S. Pat. No. 5,147,855 and published patent applications EP 0540969 and EP 0620008 which are directed to the treatment of male impotence by transdermal administration and to the treatment of neurodegenerative diseases, respectively.
VIP is a hydrophilic peptide of a very short half life in the serum (Said, S. I., Editor, Vasoactive intestinal peptide in: Advances in Peptide Hormone Research Series, Raven Press, New York, 1-512 (1982)) having the following sequence:
(SEQ ID NO:1) 1 2 3 4 5 6 7 8 9 10 11 12 His-Ser-Asp-Ala-Val-Phe-Tyr-Asp-Asn-Tyr-Thr-Arg- 13 14 15 16 17 18 19 20 21 22 23 24 Leu-Arg-Lys-Gln-Met-Ala-Val-Lys-Lys-Tyr-Leu-Asn- 25 25 27 28 Ser-Ile-Leu-Asn-NH.sub.2
To enhance its biological availability and increase its stability the present inventors have resorted to two chemical modifications reported in said patents and applications. The first was lipophilization, namely, the addition of a fatty acid moiety, designed to augment VIP's ability to penetrate biological membranes without loss of activity; thus, stearoyl-VIP, a molecule combining VIP with a stearic acid moiety at its N-terminal was designed (EP 0354992). The second modification was the replacement of native amino acids with unnatural amino acids, namely, a substitution of methionine (amino acid 17 of VIP) by norleucine, aimed at stabilizing the molecule against oxidation as well as at increasing lipophilicity; thus, stearoyl-Nle-VIP was designed (Gozes et al., Endocrinology, 134:2121-2125 (1994); Fauchere et al., Int. J. Peptide Protein Res., 32:269-278 (1988); EP 0540969). Unmodified VIP fragments derived from the 17-24 positions of the VIP sequence are described in EP 0225020 as ulcer inhibitors.
A major obstacle in the use of any substance as a medicament is its distribution in the body. The modified VIP or lipophilic VIP used for transdermal treatment of male impotence reported in the abovementioned EP 0354992 and EP 0540969 have to penetrate through the dermis and reach the erectile tissues in a short a time span as possible.
VIP, modified VIP or lipophilic VIP used to treat neurodegenerative diseases described in EP 0620008 have to pass the blood brain barrier in order to exert their therapeutic effect on brain cells.
It would have been desirable, both for the purpose of treatment of male impotence and for the purpose of administration to the CNS for the treatment of neurodegenerative diseases, to use molecules that, while having the physiological activity of the full VIP peptide, are smaller in size and thus are able to improve the bioavailability of the therapeutic compound at the target tissue. Furthermore, smaller molecules are at times more stable to degradation than larger molecules since, as a rule, they have less sites available to degradation.